The Integrated Service Digital Network "ISDN" Standard is known. The ISDN standard regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. The standard further requires that any system which desires to be ISDN compliant must be accoupled to the telephone lines, due to practical constraints; usually through a line coupling transformer that sources information on to the telephone lines within a template window. The template window specifications are defined in CCITTI.430 (I.430) of the ISDN standard.
Typically, the line coupling transformer is driven by a line driver circuit that includes four switching transistors configured as a full bridge inverter. In general, the full bridge inverter provides an alternating input to a primary winding of the line coupling transformer by enabling, during .0.1, a pair of transistors that couple to a supply voltage and a return to the primary, and, during .0.2, the other pair of transistors are enabled, such that the primary winding is coupled to supply and return in the opposite polarity of .0.1. The frequency and amplitude of the alternating input, and thus the output imposed on the secondary winding, is dictated by the transmission protocol of the ISDN standard (I.430).
Designing a line coupling circuit that includes a line coupling transformer and a line driver circuit to be compliant with the amplitude and phase requirements the I.430 specification is generally not a problem. There is a design problem, however, in complying with the transitional responses of the I.430. This problem arises because, at the trailing edge, i.e., when the alternating input is switching to the next state, the transformer and the telephone line produce a complex network that causes an oscillation, or ringing, to occur at the trailing edge. The magnitude of this ringing can easily take the switching wave form outside of the tolerance window provided by the I.430 specification.
One prior art solution to reducing the ringing at the trailing edge is disclosed in U.S. Pat. No. 4,797,904. This patent teaches momentarily providing a low impedance across the primary to effect the ringing on the trailing edge. In effect, what this patent teaches is changing the transfer function of the line coupling transformer and the telephone line during the trailing edge transition.
While this technique works well under certain load conditions, as the load on the telephone line changes so does the transfer function. As the transfer function of this circuit changes, the oscillation at the trailing edge varies. In some incidences, the oscillation varies to such a state that, even with the circuit of the U.S. Pat. No. 4,797,904, it exceeds the tolerance window dictated by the I.430 standard. Therefore, a need exists for a method and apparatus that substantially reduces the ringing on the trailing edge for varying load conditions.